The present invention relates to a fire-resistant laminated glass pane assembly comprising a glass-ceramic pane resistant to high temperatures and to thermal shocks and having rough light-diffusing surfaces, the said pane being joined, on each of its two faces, to a silicate glass pane by means of transparent intermediate layers having a refractive index corresponding to the refractive index of the glass-ceramic pane.
A laminated glass pane assembly of this type is known from the document EP-A-O 524, 418. The glass-ceramic pane has a particularly low coefficient of thermal expansion and a relatively high softening temperature and is thereby suitable, to a particular extent, for being used as fire-retarding glass. The glass-ceramic panes, are due to their extremely low coefficient of thermal expansion, so insensitive to rapid temperature variations that they even withstand the so-called extinguishing water test, during which a jet of extinguishing water is projected onto a glass-ceramic pane heated to 800xc2x0 C.
Owing to the way in which they are manufactured, however, glass-ceramic panes possess a rough surface and do not have clear transparency on account of their light-diffusing effect. So that it can be used, nevertheless, as transparent fire-retardant glazing, without a complicated grinding-down and polishing operation, the glass-ceramic pane in the known fire-retarding glass is combined optically with clear transparent silicate glass panes by means of transparent layers having the same refractive index. The surface roughness originally causing the light-diffusing effect of the glass-ceramic pane therefore does not become apparent.
The transparent intermediate layers between the glass-ceramic pane and the glass panes consist, in the known fire-retarding glass of the type described above, of an inorganic material which foams under a heat effect, in particular of hydrous sodium silicate. These intermediate layers of sodium silicate are made by casting the sodium silicate, in a form suitable for casting, containing 30 to 34% by weight of water, onto the two silicate glass panes and allowing it to dry and by subsequently joining the coated glass panes to the glass-ceramic pane in the autoclave under the effect of heat and pressure.
Fire-retarding glasses of this type have very good fireproof properties. They, nevertheless, do not have safety glass properties in terms of safety against injuries caused by splinters when the fire-retarding pane assembly breaks, for example when a person falls into a fire-retarding pane assembly of this type. In fact, the intermediate sodium silicate there does not, as such, have splinter fixation properties, as do intermediate polymer plies normally used for laminated safety glass. In many cases, however, it is desirable for fire-retarding glasses also to have safety glass properties in addition to their fireproof properties, that is to say preventing serious injuries caused by splinters when the body comes in contact with the fragmented fire-retarding glass.
Moreover, in known fire-retarding glass, the outer silicate glass panes break under shock-induced mechanical stress into large fragments which have sharp edges. Furthermore, both the two glass panes and the glass-ceramic pane have only relatively low bending strength, and this also has an adverse effect on safety glass properties because they break easily under sharp shock-induced stress.
The object of the invention is, by using a glass-ceramic pane, to develop a fire-retarding glass which has, in addition to high fire resistance, increased mechanical stability and safety glass properties against injuries caused by splinters.
According to the invention, this object is achieved in that the transparent intermediate layers between the glass-ceramic pane and the contiguous silicate glass panes consist of a thermoplastic polymer having a high splinter fixation effect, and in that the silicate glass panes are tempered thermally.
Tempered glass panes have, as such, the advantage, as a result of their compressive prestresses in their superficial regions and in the peripheral region, of possessing greatly increased tensile strength in the peripheral region and, overall, substantially higher bending strength than normal, that is to say non-tempered, glass panes. The high compressive stress in the peripheral region results in tempered glass panes having substantially increased durability in the event of fire, because the tensions, which occur in the peripheral region due to the temperature difference between the periphery sealed in the frame and the pane surface exposed to the fire, are first compensated by the compressive prestresses, in such a way that tempered glass panes do not break until much later.
Admittedly, since a polymer is used for the intermediate layer, fireproof protection is reduced, as compared with the known fire-retarding pane assembly, because the thermal insulating capacity of the polymer which carbonizes is lower than the thermal insulating capacity of the sodium silicate layer which foams. In other respects, however, this disadvantage is at least partially compensated by means of the fire-retarding pane according to the invention, due to the fact that the intermediate plies remain intact for longer on account of the longer resistance of the tempered glass panes. By contrast, the non-tempered glass panes of the known fire-retarding glazing normally collapse after fragmentation and take away with them parts of the foaming intermediate layer, so that the latter is weakened to a greater or lesser extent and consequently loses its thermal insulating capacity.
The particular advantages of the fire-retarding glass according to the invention are that, in terms of comparable fire resistance properties, it has substantially greater stability against mechanical shocks and bending stresses and, on the other hand, also pronounced safety glass properties against injuries caused by splinters.
The tempered glass panes used may, for example, be tempered glass panes made from commercial float glass. Commercial float glass has a coefficient of thermal expansion of  greater than 8.5xc3x9710xe2x88x926Kxe2x88x921 and, due to this relatively high coeffecient of thermal expansion, it may be tempered by means of conventional tempering installations such that it is capable of achieving bending strengths rising to 200 N/mm2, measured according to DIN 52303 or EN 12150. Such tempered float glass panes divide into small harmless grains of glass when they are fragmented. In other respects, however, float glass panes lose their stability owing to their relatively low softening temperature of about 730xc2x0 C., so that the thermal insulating effect of the carbonized intermediate ply is lost when float glass panes release the intermediate ply as a result of their softening. It is possible, however, to use tempered glass panes made from glasses which have a coefficient of thermal expansion xcex120xe2x88x92300 of  greater than 8.5xc3x9710xe2x88x926Kxe2x88x921, in particular of 6 to 8.5xc3x97, 10xe2x88x926Kxe2x88x921, and a softening point of 750 to 830xc2x0 C. These glasses, on the one hand, can be sufficiently tempered by means of existing tempering installations to have the required safety glass properties and, on the other hand, a markedly higher softening temperature, so that their stability is appreciably longer in the event of a fire.
The most varied polymer sheets available on the market may be used as thermoplastic intermediate layers, as long as they have the splinter fixation effect required for safety glass. The intermediate thermoplastic plies may consist either of the same or different materials. Sheets of polyvinyl butyral (PVB), polyurethane (TPU), copolymers of ethylene and vinyl acetate (EVA) or fluorinated hydrocarbons (THV) come under consideration, in particular, for this purpose. The advantage of THV sheets, in particular, is that they are hardly inflammable. In the event of a fire, virtually no inflammable decomposition product is formed when these sheets are used.